Given a binary tree, return the preorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]   1    \     2    /   3Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

难度：medium

题目：给定二叉树，返回其前序遍历结点值。（不要使用递归）

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */class Solution {    public List
    
      preorderTraversal(TreeNode root) {        Stack
     
       stack = new Stack<>();        List
      
        result = new ArrayList<>();        // root, left, right        while (root != null || !stack.isEmpty()) {            if (null == root) {                root = stack.pop();            }            result.add(root.val);            if (root.right != null) {                stack.push(root.right);            }            root = root.left;        }                return result;    }}